They're just like multiple personality disorder/dissociative identity disorder, only it's absolutely bullshit, and completely fake. Used by low-lives and losers as an excuse to like themselves, in the same vein as self-diagnosed Asperger's syndrome. Usually, these people call their "personalities" people, or headmates, or some other shit. Their alleged headmates are usually fictional characters from TV shows, which, most of the time, is some typical fandom shared by geeky teenage girls everywhere. These people fight avidly to get their headmates recognized as people, and to stop "discrimination".

On the flip side, DID is a real condition, therapist induced or otherwise. Even identities/personalities in this disorder AREN'T RECOGNIZED AS ACTUAL PEOPLE. They're psychological constructs with autonomous control of the host.

Multiple Systems: "We are a multiple system of 593,000 different headmates, from over 9000 different shows. From Goku to The Doctor to Charlie Sheen, they're all there. In my head. But, I don't have a mental disorder, I am naturally multiple. And these are all people, real ones. I am all these different people in one body."
Normal People: "Seems legit."
Multiple Systems: "You don't believe me? DISCRIMINATION!"